The present invention relates to a process for the deposition by electron cyclotron resonance (ECR) plasma deposition of electron-emitting carbon films, under the effect of an electric field applied.
Electron-emitting carbon films can be logically classified in accordance with the three following structures: diamond, DLC (diamond like carbon) and graphite. In reality, a deposited film can contain mixed structures, e.g.:
diamond crystals films with graphite inclusions,
graphite crystal films with diamond inclusions.
This classification then corresponds to the main structre.
In the prior art devices, independently of the emission of electrons, particular research has been carried out in the fields:
of synthesizing diamond films for mechanical, optical and electrical properties of the diamond, at temperatures generally between 400 and 1000xc2x0 C.,
the production of carbon films of DLC type (diamond like carbon), generally at low temperature (20 to 400xc2x0 C.) and with a high level of Cxe2x80x94C sp3 bonds, particularly for mechanical properties. Such films are mainly amorphous.
Table I, at the end of the description, reveals various devices and processes for the vacuum deposition of carbon films used at present for emissive carbon. The devices and processes considered are described in the documents given at the end of the description.
The table reveals two deposition mechanisms. A deposition of the chemical vapour deposition (CVD) type, in which a gas of organic molecules (often methane) is introduced mixed, with or without hydrogen, into a device making it possible to break the Cxe2x80x94C, Cxe2x80x94H and Hxe2x80x94H bonds by electron impact with, e.g. the use of a hot filament, the introduction of a microwave power, the use of a radio-frequency (RF) polarization or the use of an electron cyclotron resonance (ECR). As a function of the device used, the operating pressure is high (filament, microwaves, radio-frequency) or low (ECR, RF). The lower the pressure the greater the dissociation and ionization of the particles. The energy to be supplied for bringing about the transformation reaction of the gas into solid is greatly decreased by the breaks of covalent bonds (e.g. CH4) of the organic molecules. It is consequently possible to obtain crystal structures (graphite or diamond) at lower substrate temperatures (e.g. 400xc2x0 C. instead of 800xc2x0 C.). The polarization of the substrate also makes it possible to aid crystallization at a lower temperature permitting the use of a wider range of substrates.
A direct deposition of carbon ions or atoms, which can be carried out by the sputtering of a graphite target, by arc, by laser ablation, ion beam or evaporation. The quality and structure of the films are essentially dependent on the energy of the incident carbon atoms or ions for a given temperature.
The carbon films investigated for their electron emission properties by field effect in the prior art devices, correspond to one of the three following films:
A diamond crystal film with a non-negligible, graphitic, amorphous structure level (xe2x80x9cpurexe2x80x9d diamond would not appear to emit): the main problem is then the bringing about of a low temperature deposition on large surfaces. An important constraint is constituted by the use of a very small proportion (1%) of methane (or other organic molecules) compared with a large hydrogen quantity useful for the growth of the diamond, with a minimum temperature of 400xc2x0 C. It is often necessary to produce precursor sites.
A DLC carbon film with a high Cxe2x80x94C sp3 bond level: an important constraint is then the energy necessary for the incident carbon atoms or ions (approximately 100 eV) for producing such films. Certain devices, intrinsically, cannot carry out deposits over large dimensions (laser ablation, arc). This possibility has to be demonstrated for RF devices.
A varied graphite film, with or without diamond crystal inclusions.
The following technical constraints arise with such a process for the deposition of these emitting carbon films:
These electron emitting films must be implemented on large surfaces (possibly up to 1 m2) at low cost and with a good homogeneity of deposition and emission.
The temperature of the deposit must be below 600xc2x0 C. for the use of standard glass substrates.
The emission threshold field, defined as the electric field to be applied for measuring an electron current of 0.1 xcexcA, must be low (approximately 10 V/xcexcm), with a possibility of a lowering of this threshold by doping.
The carbon films must be stable during the different screen production stages, particularly during the different annealing operations, as well as during the operation of the screen, the emission of the film then being activated by an electric field.
The present invention relates to a process for the electron cyclotron resonance plasma deposition of electron-emitting material films under the effect of an electric field applied, which can be applied to large surfaces, typically of approximately 1 m2. The present invention corresponds to the last type of structure defined hereinbefore, with a Cxe2x80x94C sp3 bond level of typically 10 to 30% implemented at an average temperature of 300 to 80xc2x0 C. and at a low pressure of approximately 5xc3x9710xe2x88x924 mbars using CVD.
The present invention describes a deposition process for obtaining electron-emitting films, known that a prior substrate preparation phase must be added in order to aid the adhesion of the film or modify the type of interface bonds.
This prior phase can comprise a sputtering stage or a deposition stage with specific polarization and pressure conditions and an adequate choice of one or more gases.
The present invention relates to a process for the deposition by electron cyclotron resonance plasma of electrically conductive, electron-emitting carbon films, in which by the injection of a microwave power into a plasma chamber incorporating an electron cyclotron resonance zone, ionization takes place of a gaseous mixture under a low pressure of 10xe2x88x924 to 10xe2x88x923 mbars, the thus created ions and electrons diffusing along magnetic field lines to a substrate, characterized in that the gaseous mixture comprises organic molecules and hydrogen molecules and in that said process comprises the following stages:
heating the substrate,
creating a plasma from the ionized, gaseous mixture for a value of the magnetic field corresponding to the electron cyclotron resonance of the organic molecules,
creating a positive potential difference between the substrate and the plasma, a positive or zero polarization being applied to the substrate,
a diffusion of the plasma to the substrate, said substrate having reached, by heating, a temperature such that the electron-emitting material is deposited on the substrate.
In an advantageous embodiment these different stages are performed simultaneously.
Advantageously, the electron-emitting material is graphitic carbon with a minority proportion of sp3 bonds. The substrate temperature is advantageously between 300 and 800xc2x0 C.
The carbon films are advantageously constituted by a graphite matrix with inclusion of diamond crystals.
The substrate can be heated by electron bombardment or by an external heating device. The organic gas can be methane, methanol, acetylene, etc.
Advantageously the microwave power is injected at a frequency of 2.45 GHz. The substrate is positively polarized between typically +50 and +200 volt. The plasma is advantageously polarized by the plasma chamber at xe2x88x9250 volt, the substrate being earthed or grounded.
It is possible to vary the proportion of organic molecules compared with the hydrogen in the gaseous mixture by a few % to 100%.
The substrate can be of silicon, silicon covered with a conductive film, e.g. of molybdenum, or standard glass covered with a conductive film, e.g. of molybdenum.
One of the main applications of the process according to the invention is that of producing large size, flat, field effect screens, where the carbon acts as an electron source, advantageously replacing metallic micropoints.